Fuel cell system

Abstract

To provide a fuel cell system configured to appropriately measure the AC impedance of a fuel cell. A fuel cell system wherein a controller controls ON and OFF of switches of n phases; wherein the controller monitors current values of coils; the controller operates the switches of the n phases at different phases; wherein the controller operates duty ratios of the switches of the n phases with periodically increasing and decreasing them, and the controller measures an AC impedance of a fuel cell from a current waveform of and a voltage waveform of the fuel cell; and wherein, when the controller determines that a predetermined condition 1 is met, the controller makes amplitudes which increase and decrease the duty ratios large compared to other operating conditions.

The invention claimed is: 
     
       1. A fuel cell system,
 wherein the fuel cell system comprises a fuel cell and a converter for carrying out at least one selected from the group consisting of step-up and step-down of output voltage of the fuel cell; 
 wherein the converter comprises:
 a reactor comprising coils of two phases, which are magnetically coupled to each other, 
 switches of the two phases, which are connected to the coils, and 
 a controller; 
 
 wherein the controller controls ON and OFF of the switches of the n phases; 
 wherein the controller monitors current values of the coils; 
 wherein the controller operates the switches of the two phases at a phase difference of 180°; 
 wherein the controller operates duty ratios of the switches of the two phases with periodically increasing and decreasing them, and the controller measures an AC impedance of the fuel cell from a current waveform of and a voltage waveform of the fuel cell; and 
 wherein, when the controller determines that the following condition 1 is met, the controller makes amplitudes of the current values of the coils which increase and decrease the duty ratios large compared to other operating conditions: 
 Condition 1: an operating condition in which, when the switches of the two phases are operated at a phase difference of 180°, and when currents flowing through the coils are in a discontinuous mode and the duty ratios of the switches of the two phases are less than 50% (D <0.5) and satisfy any one of the following formulae (A) and (B):
     D <{(½)( L−M )( V   H   −V   L )}/( LV   L   +MV   L   −MV   H )  Formula (A)
 
     D <(½){1−( V   L   /V   H )}  Formula (B)
 
 
 where L is self-inductance of the reactor; M is mutual inductance of the reactor; V H  is an outlet voltage of the converter; V L  is an inlet voltage of the converter; and D is the duty ratio (-). 
 
     
     
       2. The fuel cell system according to  claim 1 ,
 wherein the controller preliminarily stores a data group showing a relationship between the inlet voltage of the converter, the outlet voltage of the converter, the duty ratios of the switches and the current values of the coils, and 
 wherein, when the controller determines that the condition 1 is met, the controller refers to the data group and makes the amplitudes which increase and decrease the duty ratios of the switches of the two phases, large compared to other operating conditions. 
 
     
     
       3. The fuel cell system according to  claim 1 ,
 wherein the controller confirms whether or not there is a request for AC impedance measurement of the fuel cell; 
 wherein, when the controller confirms that there is a request for AC impedance measurement of the fuel cell, the controller determines whether or not the condition 1 is met; and 
 wherein, when the controller determines that the condition 1 is met, the controller makes the amplitudes which increase and decrease the duty ratios of the switches of the two phases, large compared to other operating conditions, and the controller measures the AC impedance of the fuel cell. 
 
     
     
       4. The fuel cell system according to  claim 1 ,
 wherein the input voltage (V L ) into the converter is 20 V and the average current value of the coils flowing through the reactor is 9 A; 
 wherein, when the controller determines that the condition 1 is not met, the controller is configured to increase and decrease the duty ratios in a range of from 23% to 33%, the controller is configured to increase and decrease the current values of the coils between 6 A and 12 A, and the amplitudes of the currents are ±3 A; and 
 wherein, when the controller determines that the condition 1 is met, the controller is configured to increase and decrease the duty ratios in a range of from 14% to 38%, the controller is configured to increase and decrease the current values of the coils between 4 A and 14 A, and the amplitudes of the currents are ±5 A.